The invention relates to optoelectronic semiconductor devices, circuits, and waveguides, and, more particularly, to semiconductor lasers integrated with dielectric waveguides and methods of fabrication.
Optical communication systems typically employ semiconductor laser sources and glass optical fiber communication channels. Semiconductor lasers in the form of heterojunction diodes with quantum well active regions and made of materials such as Al.sub.X Ga.sub.1-X As with GaAs quantum wells provide a compact and rugged source of infrared light which can be easily modulated by simply varying the diode current. In particular, a stripe geometry diode laser may be as small as 10 .mu.m wide by 100 .mu.m long by 30 nm thick active area imbedded in a 400 nm thick cavity. The reflecting ends of the cavity may be distributed Bragg reflectors to avoid cleaved mirror ends. For example, Tiberio et al., Facetless Bragg reflector surface-emitting AlGaAs/GaAs laers fabricated by electron-beam lithography and chemically assited ion-beam etching, 9 J.Vac.Sci.Tech.B 2842 (1991), discloses 250 .mu.m wide diodes of Al.sub.X Ga.sub.1-X As with GaAs quantum wells and distributed Bragg reflector ends made of free standing (on the same semiconductor substrate as the diode) AlGaAs fingers spaced with a period of 120 nm. Further, Tiberio et al. formed a grating in the AlGaAs with period 307 nm adjacent to the distributed Bragg reflector; the grating launched the laser output into space at a 45 angle to the surface of the substrate containing the laser with distributed Bragg reflector. See FIG. 1 which shows the Tiberio et al. setup in cross sectional elevation view.
Tennant et al., Multiwavelength distributed Bragg reflector laser array fabricated using near field holographic printing with an electron-beam generated phase grating mask, 11 J.Vac.Sci.Tech.B 2509 (1993), describe the use of holographic lithography for patterning distributed Bragg reflectors in an array of seimconductor lasers for use with wavelength division multiplexing.
The output of a stripe heterojunction diode semiconductor laser, typically operated in the lowest order TE mode, has a highly asymmetric near field pattern (see end on view of FIG. 2a) and a wide vertical spread angle (numerical aperture .sup..about. 0.35, see elevation view of FIG. 2b) but a small horizontal spread (numerical aperture .sup..about. 0.1) due to the typically large difference in index of refraction between the active core and the cladding and the small height to width ratio of the cavity. In contrast, communication channel optical fibers which would carry the output of the semiconductor laser has a circular core (see end on view of FIG. 2c) and a small spread angle due to the small difference between the index of refraction of the core and the cladding (see elevation view of FIG. 2d); that is, the numerical aperture is symmetric and small (.sup..about. 0.15). Thus the direct coupling of the semiconductor laser output into an optical fiber has low efficiency due to mode field mismatching, and the usual approach to this problem inserts a lens between the laser and the optical fiber or to form the fiber end into a lens. FIG. 3a shows such a lens insertion to provide a high efficiency coupling, and FIG. 3b shows an optical fiber with its end rounded into a lens. Brenner et al, Integrated Optical Modeshape Adapters in InGaAsP/InP for Efficient Fiber-to-Waveguide Coupling, 5 IEEE Phot.Tech.Lett. 1053 (1993), describe a semiconductor waveguide with vertically tapered core and thickened cladding at a waveguide end for better coupling to a lensed optical fiber.
However, such inserted lens or lensed optical fiber end systems increase costs of fabrication and assembly and decrease ruggedness of the final system.
Further, wavelength division multiplexing requires combining outputs of discrete lasers using an optical fiber multiplexer which is bulky and costly.